textproduct: Rapid City

KEY MESSAGES

- Showers taper off tonight as some low clouds/fog linger - Drying out for the rest of the week, but southwest flow aloft will bring limited chances for shra/TS - Breezy/windy spots Tuesday/Wednesday

DISCUSSION

(This Evening Through Monday) Issued at 109 PM MDT Mon Sep 21 2026

19z surface analysis had sprawling high pressure over the Great Lakes into the northern Plains bringing low-level easterly component to boundary layer flow. Water vapour loop had upper low spinning over the SD/NE/IA/MN border area with backwash moisture spilling into the CWA. This moisture combined with weak upslope supporting a few showers/drizzle over the area this afternoon, particularly in/near Black Hills per latest KUDX radar loop. Effects of upper low exiting will be the main forecast concern mid-week.

Tonight through Wednesday, upper low shifts away with upper ridge expanding into the northern Plains. Southwest flow aloft develops over the area with a few weak shortwaves expected to saunter through the CWA. First one arrives early Tuesday, helping increase gradient between developing lee trough and departing surface high. Two effects will occur. Breezy/windy spots will develop Tuesday with increasing southeasterly winds. This will also support some fog/low stratus, especially east of the Black Hills. Earlier NBM guidance had bonkers wind gusts, but latest SPC HREF/NBM support a few gusts to 35-45mph from northwest into south-central SD with <20% chance of >45mph wind gusts. Other effect will be small clusters of PoPs southeast of the Black Hills given weak theta-e advection. Stronger wave moves northeast from the central Rockies into MT Tuesday night/Wednesday. Effect of this wave more pronounced with stronger low-level jet/theta-e advection. This should promote some shra/TS moving southwest to northeast over the CWA as MUCAPE slowly increases, especially over northeastern WY. Temperatures will be near guidance and warmer overall.

Warmer/drier weather expected the rest of the forecast period per southwest flow aloft and periodic low level downslope flow. The chance of precipitation is not zero, however.

AVIATION

(For the 18Z TAFS Through 18Z Tuesday) Issued At 1121 AM MDT Mon Sep 21 2026

Low clouds will continue to linger through the day. MVFR/IFR conditions are expected across the SD plains, with some potential BR and IFR/LIFR ceilings early Tuesday morning lasting through the end of the forecast period. Far northeastern WY will also see lowered ceilings Tuesday morning but they should remain MVFR/IFR and burn off later Tuesday morning.

UNR WATCHES/WARNINGS/ADVISORIES

SD...None. WY...None.